The present invention relates to a device for transmitting and/or receiving electromagnetic waves, more particularly to an antenna known by the expression xe2x80x9cprinted antennaxe2x80x9d fed from an array produced in mircostrip technology.
Hereinbelow, the expression xe2x80x9cprinted antennaxe2x80x9d (or xe2x80x9cmicrostrip antennaxe2x80x9d) will refer to an antenna produced in so-called xe2x80x9cmicrostripxe2x80x9d technology, comprising a radiating element, typically a xe2x80x9cpatchxe2x80x9d, a slot, a dipole, etc., or an array of such elements, the number of elements depending on the desired gain. This type of antenna is used as primary source at the focus of a lens or of a parabola or as a planar array antenna.
In printed antennas, the radiating elements, be they unitary or grouped into an array, are fed from a feed array formed of microstrip lines. In general, this feed array radiates, to a greater or lesser extent, undesired radiation or parasitic radiation which disturbs the main radiation of the antenna. The principal effects resulting from this parasitic radiation are a rise in the cross-polarization of the printed antenna. Other undesirable effects, which are more or less significant, may also result from this parasitic radiation, namely:
an impairment of the radiation pattern of the antenna with a rise in the side lobes and/or a deformation of the main lobe,
an impairment of the efficiency of the antenna, namely radiation losses.
Current solutions attempt to limit or minimize the parasitic radiation:
through a judicious choice of the parameters of the dielectric substrate such as the thickness, permittivity, etc.,
by optimizing the line widths,
or by minimizing the discontinuities from which the parasitic radiations stem.
However, all the solutions proposed hitherto require compromises which limit their effectiveness. For example, a slender substrate exhibiting a high dielectric permittivity minimizes the radiation of the feed lines but also reduces the effectiveness of the radiation of the radiating elements and hence the efficiency of the antenna. Likewise, the use of narrow lines reduces the parasitic radiation but the smaller the widths of the lines, the larger the ohmic losses.
Consequently, the aim of the present invention is to propose a solution which, instead of reducing the harmful effects of the parasitic radiation, uses them to contribute to the main radiation of the antenna.
A subject of the present invention is therefore a device for transmitting and/or receiving electromagnetic waves comprising an antenna with at least one radiating element transmitting and/or receiving signals of given polarization and a feed array produced in microstrip technology consisting of lines devised so as to give parasitic radiation, characterized in that the feed array is devised and dimensioned in such a way that the parasitic radiation has the same direction and the same polarization as the radiation of the antenna and combines in-phase with the said radiation of the antenna.
In a known manner the parasitic radiation is generated by discontinuities in the lines of the feed array, such as elbows, T circuits, line width variations.
In accordance with one embodiment of the present invention, the relative phase of the source of parasitic radiation is determined by the length of the lines of the feed array. Preferably, the feed array is a symmetrical array.
In the case of a linearly polarized antenna, the lengths of lines Li on each side of an elbow are given by the following equations:
L1=xcex{fraction (1/2)}+k1xcex1 k1=0,1,2, . . . 
L2=k2xcex2 k2=0,1,2, . . . 
where xcexi represents the wavelength guided in the line of the feed array of length Li with:
xcexi=30/(f{square root over (xcex5r eff)}) [in cm]
with f: working frequency [in GHz]
xcex5r eff: effective permittivity of the material for the portion of line of length Li.
Moreover, in the case of a circularly polarized antenna comprising at least two radiating elements, the lengths of lines Li of the feed array formed of a T circuit with two elbows are given by the following equations:
Lxe2x80x22=L2+k1xcex{fraction (2/4)}k1=1,2,3
where Lxe2x80x22 and L2 are the two branches of the T.
Lxe2x80x23=L3+k2xcexxc2xek2=1,2,3
where L3 and Lxe2x80x23 are the lines connecting to the radiating elements.